In recent years and continuing, protection of the global environment and energy savings are becoming major concerns. Accordingly, fuel batteries and solar batteries, which have low impacts on the environment, are increasingly adopted as power supplies for portable devices. With the implementation of digital terrestrial broadcasting and next generation communication standards, the amount of power consumed by mobile phones is particularly increasing, which reduces the battery standby time. Furthermore, laptop computers are required to have longer continuous operation times. Thus, low-power-consumption devices and high-energy-density batteries are desired for addressing such issues.
Solar batteries and fuel batteries have higher energy density compared to lithium-ion batteries; however, their power generating voltage per cell is generally low, at 0.3 V-1.23 V, and thus cannot drive a load with such a low voltage. Furthermore, as solar batteries and fuel batteries have extremely low output density, a so-called hybrid configuration is employed. Specifically, in such a hybrid configuration, the power generating voltage is boosted by a booster circuit, the boosted voltage is stored in a secondary battery that is connected in parallel and has a high-output-density, and the power generating voltage is supplied from the secondary battery to the load. However, it is extremely difficult to drive the booster circuit and a control circuit thereof with such a low voltage.
One approach is to provide a storage unit to supply voltage for driving the control circuit of the booster converter. First, power is supplied from the storage unit to the control circuit to activate the booster converter, and after the booster converter is activated, output from the booster converter is used to drive the control circuit. Subsequently, a switch and a countercurrent preventing unit are used to separate the storage unit from the load and the control circuit. Accordingly, a low-voltage direct current power supply can be used as the main power supply for driving the load, and the capacity of the storage unit can be small. This technology makes it possible to drive the load even if the power generating voltage is lower than the voltage required for activating the booster circuit (see, for example, Patent Document 1).
Patent Document 1: Japanese Laid-Open Patent Application No. 2004-147409
However, in the above-described technology, when a portable device is abandoned for a long time without output voltage from the main power supply, an electrical current slightly leaks from the switch and the countercurrent preventing unit, or a natural self-discharge of the storage unit is caused. Therefore, the voltage of the storage unit drops to a level at which the control circuit cannot be activated. As a result, the booster converter cannot be activated, power cannot be supplied from the main power supply to the load, and the device becomes permanently unusable.